Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Sep. 30, 2021
Deferred tax assets:
Employee benefit plans	$ 57,094	$ 64,316
Net operating loss carryforwards	485,061	911,424
Charitable and other credit carryforwards	1,903	7,712
Regulatory excess deferred tax	110,548	148,200
Lease asset	50,007	52,138
Other	44,035	33,591
Total deferred tax assets	748,648	1,217,381
Valuation allowance	(523)	(663)
Net deferred tax assets	748,125	1,216,718
Deferred tax liabilities:
Difference in net book value and net tax value of assets	(2,431,757)	(2,258,264)
Gas cost adjustments	(43,964)	(26,413)
Winter Storm Uri regulatory asset	(20,710)	(471,025)
Lease liability	(50,007)	(52,138)
Rate deferral adjustment	(49,309)	(47,445)
Interest rate agreements	(106,820)	(20,156)
Other	(45,063)	(47,086)
Total deferred tax liabilities	(2,747,630)	(2,922,527)
Net deferred tax liabilities	$ (1,999,505)	$ (1,705,809)